Process of treating metal surfaces



Oct. 5, 1937. H. M. S ITH 2,095,105 I PROCESS OF TREATING METAL SURFACES Filed Oct. 1, 1935 Invento r: Henry M. Smith, b V v fflaw-z r y His 2 ttorney.

Patented Oct. 5, 1937 PROCESS OF TREATING METAL SURFACES Henry M. Smith, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application October 1, 1935, Serial No. 43,021

6 Claims.

My invention relates to a process for treating metal surfaces to give them increased smoothness and uniformity, and more particularly to a process for polishing filamentary heating members for use in lamps and electric discharge devices.

The temperature at which a tungsten filament for a lamp, vacuum tube, or similar discharge device, will operate for a given filament current is dependent in part upon the condition of the a surface of the wire. In general, the smoother the surfaceof the wire, the higher will be the temperature at which it will operate in vacuum for a, given diameter and filament current. Since the electron emission of a filament is extremely sensitive to variations in its operating temperature, it is of great importance in electric discharge devices that the filament temperature shall not fall below its designed value because of imperfections in the material used. Furthermore, to satisfy the commercial requirements it is essential that the method of treatment adopted to obtain this result be such as to insure a highly uniform product.

While a filament surface of the desired quality may be obtained by electrolytic means, it is believed at present to be impracticable to use such a method of treatment without producing some reduction in the wire diameter and a consequent variation in the operating characteristics of the filament. It is, therefore, an object of my present invention to provide a method of smoothing tungsten filaments and other metal surfaces which will cause no decrease in the dimensions or material content -of.the object treated.

For attaining this result I produce on the surface of a particular metal body a compound or alloy of the metal having a lowermelting point than the metal itself. The compound or alloy is then heated and caused to flow evenly over the surface in question and is finally reconverted into pure metal. In this way a perfectly even surface is obtained withoutmodifying in any way the other physical characteristics of the metal body.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, will best be understood by reference to the following specification taken in 7 discharge devices, but it should be understood to be in no way limited thereto. In applying my invention to the conditions of actual manufacturing practice, I prefer first to clean thoroughly the surface of the wire to be treated by flashing it in hydrogen at a temperature above about 2700 C. This may conveniently'be accomplished by mounting the wire (shown in the drawing as a coiled filament l) within the confines of an inverted cup or gas dome 2. To maintain the proper atmosphere a continuous stream of hydrogen from a source such as tank 3 may be supplied at an orifice 4 at the top of the dome and. permitted to exhaust to atmosphere at the bottom thereof. The high temperature stipulated above may be obtained by impressing on the filament terminals a suitablepotential from a source 5 of direct or alternating current. Regulation of the magnitude of the impressed voltage may be achieved by an appropriate adjusting circuit exemplified in the present instance as a potentiometer connection 6 cooperating with a resistance .I shunted across the terminals of the voltage source. At the temperature stated the cleaning operation ordinarily will require only a very short time, not in excess of 3 or 4 seconds.

Following the cleaning operation the surface of the filament I should be carbonized. This operation maybe carried out according to any of the well known methods of the prior art but is most conveniently accomplished by means of the apparatus illustrated in the drawing. To this end valves 8 and 9 are adjusted so that the former is open and the latter closed so as to cause the hydrogen stream to pass over the surface 9 of a quantity of volatile hydrocarbon, such as benzol, contained in the tank ID. A s'uflicient quantity of the benzol will be retained by the hydrogen to supply an adequate carbon-containing atmosphere for the dome 2. The amount of hydrocarbon transmitted may be regulated invariou's ways as by adjusting the level of the liquid surface 9' with respect to the tubu-' lar orifice II. I

.The carbonizatlon step is carried out with the filament at a temperature of from about.1200 to about 1500 C., this factor being regulated as above explained by adjustment of the potential impressed on the filament I. Under the conditions stated .the carbonization process will result in the production of the various carbides of tungstenon the filament surface. In'so far as .the nature of such carbides is essential'to the purposes of my invention, they are characterized by having melting points as low as 2777 and 2877 C. as compared with 3370" 0., the melting point of tungsten. After carbonization hasproceeded to a sufilcient degree, the valve 9 may be opened and the valve 8 closed, so that the filament is again immersed in a pure hydrogen atmosphere. Under such conditions the temperature may be raised to a point between the melting point of tungsten carbide and that of the tungsten core. At a temperature in excess of about 10 2877 C. the carbide coating becomes fused and will flow over the base metal so as to form a perfectly uniform. and smooth surface. I have found thatfour heating periods of from to 1 second each are adequate toobtain a good smoothing effect. It is possible, however, to obtain a somewhat greater consistency of results by repeating the carbonizing and melting operations one or more times.

After the melting step has been properly carried out, the filament surface may be completely decarbonized by continued flashing in hydrogen at a temperature approaching that of tungstens melting point. The removal of the carbon results in the restoration of a pure tungsten surface modified only to the extent of having all irregularities and variations eliminated.

While I have described my improved process as being particularly applicable to the treatment of tungsten filaments, it will be understood to be equally useful in many other situations in which an extremely smooth metal surface is to be desired. In general, its practice depends upon the production on the surface to be treated of a compound or alloy of the base metal having a melting point below that of the metal itself.

In the case of iron and nickel the preliminary steps may be carried out by causing a layer of the sulphides or phosphides of the metal in question to be formed on the surface under treatment.

For forming the sulphide layer it is possible to use an atmosphere such as a mixture of hydrogen and hydrogen sulphide, while the phosphide coatings may be procured by the use of a hydrogenphospho'rus compound such as phosphine. The

5 layers thus formed may be reconverted to the base metal by being subjected to the reducing action of hydrogen at an elevated temperature as previously explained. Since both the coating compounds mentioned have a lower melting point than the metals themselves, it will be seen that they are amenable to the smoothing treatment described. It is only essential that the melted material have a surface tension and coemcient ofadhesionsuchastoinsurethatitwillwet or flow evenly over the surface of the metal to be polished.

I have found it to be characteristic of my improved process that the surfaces in connection with which it is used are reduced to a condition so of almost perfect uniformity and smoothness. Even the fine die marks, which are otherwise removable only with the greatest diiliculty, are found to be entirely eliminated. At the same time no slightest reduction in the diameter or volumetric content of the metal body can be observed.

While I have described a particular succession of operations as exemplary of my invention, it

will of course be understood that I do not wish 1. The method of smoothing a metal surface which comprises forming on said surface a compound of said metal having a lower melting pointthan the metal itself, heating said compound to a temperature intermediate between the melting points of said compound and said metal and thereafter chemically reconverting said compound to said metal. 7

2. The method of smoothing a tungsten surface which comprises forming on said surface a tungsten compound having a lower melting point than tlmgsten itself, heating said compound to a temperature, intermediate between the melting points of tungsten and of said compound, and thereafter chemically reconverting said compound to tungsten.

3. The method of smoothing a tungsten surface which comprises forming tungsten carbide on said surface, heating said surface to a temperature between about 2800 andabout 3300 C. thereby to cause said carbide to fiow evenly over said surface and thereafter chemically converting said tungsten carbide to tungsten.

4. The method of polishing a tungsten filament which comprises carbonizing the surface cf said tween 2800 C. and 3300" C; and thereafter de-' carbonizing said surface by heating in a hydrogen atmosphere.

6. The method of smoothing a metal surface which comprises forming on said surface a substance having said metal as an ingredient thereof and having a lower melting point than the metal itself, heating said substance to a temperature intermediate between the melting points of said metal and said substance thereby to cause the substance to fiow evenly over said surface and thereafter bringing said surface into contact with an atmosphere eflective to reconvert saidv 1 substance to said metal.

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